1. Field of the Invention
The present invention relates to an engine exhaust gas recirculation system effective to reduce the emission of nitrogen oxide (NO.sub.x) from an exhaust system of an internal combustion engine.
2. Description of the Prior Art
There has been hitherto known an exhaust gas recirculation system (hereinafter referred to also as E.G.R. system in abridgement) for internal combustion engines which comprises an exhaust gas recirculating passage for tapping the engine exhaust gas from an exhaust pipe of the engine and feeding back to an intake pipe thereof at a location downstream of a throttle valve, a flow control valve provided in the recirculating passage for controlling the flow of the exhaust gas fed back to the intake pipe in response to a pressure signal derived from the engine intake pressure in the intake pipe, and a pressure control valve apparatus for correctively controlling the pressure signal to be transmitted to the flow control valve as a function of the pressure in a pressure cell which is provided in the recirculating passage between the exhaust gas pipe and the flow control valve.
With the arrangement of the hitherto known E.G.R. system outlined above and described hereinafter in detail, the quantity of the so-called external exhaust gas recirculation which is caused to flow back to the intake pipe from the exhaust pipe through the recirculating passage can be maintained at a predetermined ratio with respect to the quantity of engine intake air. However, with the prior art E.G.R. system, it has been impossible to control the quantity of the recirculated exhaust gas in consideration of the operating load conditions of the associated internal combustion engine.